Solenoid operated valves and pumps are driven in their simplest form by a coil and an armature that is free to move within the coil. The armature is normally spring loaded away from the energized position such that when a power pulse is applied to the coil, the armature is pulled into the energized position and in moving opens or closes the valve. It is known that once the solenoid has moved to the end of its operating stroke, no further work is done by the armature.
The amount of current flow through the coil determines the strength of the magnetic field acting upon the armature and the voltage applied to the coil determines the current flow through the coil. The duration of voltage application to the coil must be sufficiently long in order to permit the armature to complete its operating stroke. After the operating stroke has been completed, the current through the coil can be reduced to the amount of current necessary to hold the armature in place. This current is called the hold current. Current in excess of the hold current wastes power and reduces valve life.
In order to efficiently control the solenoid, the voltage waveform to drive the coil (i.e., a drive voltage waveform) is typically selected to provide sufficient power to drive the solenoid efficiently. The prior art requires extensive manual calibration and testing in order to find and tune a ‘suitable’ or optimum drive voltage waveform for a particular valve. In other words, ‘plug and play’ of the valves is not feasible. This is due to several reasons.
One reason is that the drive voltage may be fixed in operation. When the drive voltage is fixed in operation, the drive is in principle sub-optimal in operation because there is unit-to-unit variation of the valve electromagnetic and mechanical parameters.
Another reason is that there is also a very strong type-to-type variation. For example, the pull time, pull current, hold current and closure point can be significantly different between different manufacturer's valve for the same application. The prior art does not allow a simple replacement of one type for another without repeating the extensive manual calibration. For example, one cannot simply remove a valve manufactured by a valve manufacturer and install a valve manufactured by another valve manufacturer and vice-versa without repeating the manual calibration step.
Another reason is that the closure point detection (i.e., detecting when the solenoid closes) information from prior systems is not reliable. In these systems, a numerical algorithm detects closure by finding an inflection point in the current feedback from the coil. The current feedback signal typically exhibits several ‘non-linearities’ (e.g., inflections). In order to differentiate these from the closure point, the drive signal is compromised and the search window used to find the closure point has to be very narrowly defined. Additionally, finding inflections in a signal is very sensitive to noise. As a result, this technique is sensitive to cycle-to-cycle variation and unit-to-unit variation.